Enhanced concurrent access

• Concurrent. An application runs on all active cluster nodes at the same time.

  To allow such applications to access their data, concurrent volume groups are varied on for all active cluster nodes. The application has the responsibility to ensure consistent data access.

• Nonconcurrent. An application runs on one node at a time.

  The volume groups are not concurrently accessed. They are still accessed by only one node at any given time.

When you varyon the volume group in enhanced concurrent mode on all nodes that own the resource group in a cluster, the LVM allows access to the volume group on all nodes. However, it restricts the higher-level connections, such as NFS mounts and JFS mounts, on all nodes, and allows them only on the node that currently owns the volume group in PowerHA SystemMirror.

You can use the AIX® MPIO function to access disk subsystems through multiple paths. Multiple paths provide more throughput and higher availability than the use of a single path. In particular, when multiple paths are used, failure of a single path due to an adapter, or a cable or switch failure will not cause applications to lose access to data. While PowerHA SystemMirror will attempt to recover from complete loss of access to a volume group, that loss itself is going to be temporarily disruptive. The AIX MPIO function can prevent a single component failure from causing an application outage.

When fast disk takeover is used, the disk reservation function is not used. If the cluster becomes partitioned, nodes in each partition could accidentally varyon the volume group in active state. Because active state varyon of the volume group allows mounting of file systems and changing physical volumes, this situation can result in different copies of the same volume group. For more information about fast disk takeover and using multiple networks, see the section Using fast disk takeover.

Concurrent access requirements for MPIO accessed disks

Enhanced concurrent mode is the only option for creating concurrent volume groups. In PowerHA SystemMirror, enhanced concurrent mode volume groups do not use disk reserves. The concurrent access that is required for MPIO accessed disks is automatically provided in PowerHA SystemMirror.

About enhanced concurrent mode

All concurrent volume groups are created as enhanced concurrent mode volume groups by default. For enhanced concurrent volume groups, the Concurrent Logical Volume Manager (CLVM) coordinates changes between nodes through the Group Services component of the Reliable Scalable Cluster Technology (RSCT) function in the AIX operating system. Group Services protocols flow over the communications links between the cluster nodes.